linux-kernel - Re: [PATCH 21/22] arm64: replace ZONE_DMA with ZONE

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Message-ID: <3c3263ca-b0ab-654a-d67d-f5ff5e31280c@arm.com>
Date:   Wed, 10 Jan 2018 17:10:02 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org, linux-arch@...r.kernel.org,
        linux-mips@...ux-mips.org, Michal Simek <monstr@...str.eu>,
        linux-ia64@...r.kernel.org,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad@...nok.org>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 21/22] arm64: replace ZONE_DMA with ZONE_DMA32

On 10/01/18 15:55, Christoph Hellwig wrote:
> On Wed, Jan 10, 2018 at 04:55:17PM +0100, Christoph Hellwig wrote:
>> On Wed, Jan 10, 2018 at 12:58:14PM +0000, Robin Murphy wrote:
>>> On 10/01/18 08:09, Christoph Hellwig wrote:
>>>> arm64 uses ZONE_DMA for allocations below 32-bits.  These days we
>>>> name the zone for that ZONE_DMA32, which will allow to use the
>>>> dma-direct and generic swiotlb code as-is, so rename it.
>>>
>>> I do wonder if we could also "upgrade" GFP_DMA to GFP_DMA32 somehow when
>>> !ZONE_DMA - there are almost certainly arm64 drivers out there using a
>>> combination of GFP_DMA and streaming mappings which will no longer get the
>>> guaranteed 32-bit addresses they expect after this. I'm not sure quite how
>>> feasible that is, though :/
>>
>> I can't find anything obvious in the tree. The alternative would be
>> to keep ZONE_DMA and set ARCH_ZONE_DMA_BITS.
>>
>>> That said, I do agree that this is an appropriate change (the legacy of
>>> GFP_DMA is obviously horrible), so, provided we get plenty of time to find
>>> and fix the fallout when it lands:
>>>
>>> Reviewed-by: Robin Murphy <robin.murphy@....com>
>>
>> I was hoping to get this into 4.15.  What would be proper time to
>> fix the fallout?
> 
> Err, 4.16 of course.

Hee hee - cramming it into 4.15 is exactly what I wouldn't want to do, 
even if Linus would accept it :)

Landing it this merge window for 4.16-rc1 sounds good if we can manage that.

Robin.